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Plant Of The Week #6: Dioscorea polystachya

This week I would like to share with you a hardy, perennial vine that I learned about earlier last year and at some point (hopefully sooner rather than later) would like to add to my living collection of useful and beautiful plants. Although drab by most gardener’s standards, this vigorous and fast growing plant definitely deserves some space, especially if you are interested in unusual yet perfectly practical plants that, really, just haven’t quite caught on yet. Diversity is strength, and never is that more true than in your garden or on your plate. Your 6th Plant of the Week is Dioscorea polystchya.

Dioscorea is a predominately tropical or sub tropical genus although there are many species including D. polystachya that grow naturally in temperate climates with distinct cool or cold winters. Many of the plants in this group are perennial vines (technically lianas) which produce long, starchy, smooth skinned tubers as well as small ‘bulblets’ which are easily detached from the plant. These bulblets (which look remarkably like miniature potatoes) can develop into an entirely new plant if they happen to find themselves in a favorable spot and therefore aid in vegetative dispersal. The actual seeds of the plant are only produced on female plants and are winged seeds which are contained within flat 3-sides capsules which are born in clusters on a thin, dangling stem.

Before I get into the specifics, I would like to clear up a few discrepancies that I came across during the editing of this post. The focal species for this article is Dioscorea polystachya, which has several obsolete synonyms that are still commonly used in place of the officially recognized name. Dioscorea batatas as well as D. opposita are both names that are sometimes found to be referencing D. polystachya, which in English is known most often as the Chinese yam. Confusingly, D. opposita is also used incorrectly in place of D. oppositifolia. This is the official name for a species that is indigenous to the Indian subcontinent and does indeed have leaves that grow in pairs.

D. japonica is a very similar species to D. polystachya and the two plants are both extensively cultivated in their native lands of east Asia and by adventurous and curious gardeners elsewhere in the world where the two species can be successfully grown and yield well. The Japanese yam, D. japonica (also known as the glutinous yam as the refined starch from the root is often used to thicken soups), is likely not quite as hardy as D. polystachya which is hardy to zone 4. D. alata, the purple yam (although there are also varieties that are white) is a species that is believed to have originated in south East Asia but is now grown throughout the world including South America and Africa and in some of these regions is considered a staple crop.*

In Canada there is only one native Dioscorea, often known simply as the wild yam, D. villosa. This species does not produce aerial bulblets but does grow a tuberous root, however the root was utilized by Native American cultures almost exclusively as a medicine. Although there isn’t (or at least from my end there doesn’t appear to be) any actual scientific medical evidence to back up these claims, the roots supposedly contain diosgenin which is used to artificially produce progesterone and steroids. The root is also thought to be anti-inflammatory, anti-spasmodic and was used to treat ills as varied as menstrual or gall bladder pains, arthritis, irritable bowel syndrome and nausea. Certainly a native medicinal plant with this wide of a scope of potential uses ought to be thoroughly investigated.

Dioscorea polystachya, like most of it’s relatives, produces a remarkably large tuber that can grow to several feet in length if grown over the course of several growing seasons and harvested whole. It is often roughly circular in diameter with a thin light brown skin sporting a sparse coating of long root hairs which conceals a pearly white interior. The roots are quite delicate and so great care and patience is required when the roots are to be harvested. Some gardeners grow yams in tall raised beds which removable wooden slats so as to make this task easier as well as more gentler on the tubers.

The tubers are quite starchy and can be baked, boiled, mashed or fried in the same way that one would prepare potatoes. The roots tolerate long term storage well as long as conditions are humid and cool, and so can be harvested and eaten slowly over the winter months. During the summer, small starchy bulblets or tubercles develop in the axils of the leaves and these can be collected once they easily detach from the vines which is usually in early autumn as the leaves may be changing color. These can be cooked and eaten in soups, stews or fried or they can be planted in new situations in order to propagate the species.**

Since the Chinese yam is a vine, successfully growing it will require a trellis of some sort so as to prevent D. polystachya from becoming a hopelessly tangled mess and sprawling over the lawn or engulfing and eventually killing nearby shrubs, hedges or trees. Planting underneath of a dead tree works wonderfully, as does training them along a fence or up a wall. They could also be grown like sweet potatoes (Ipomoea batatas) by planting them in mounds and allowing them to sprawl across the soil in between them, although this method uses up quite a bit of space.

The vines will grow best and positively thrive in full or near full sun in a variety of different soils as long as they are rich and well drained. The vines are also perennial yet herbaceous, so the tender vines that are produced from the roots every spring will die back to the ground after the first few hard frosts of winter. Therefore the dried, withered stalks can be removed at the end of each growing season. Ideally the roots are harvested after their second and third years, with the tubercles used as planting stock for future harvests. If some tubercles are planted every year once your first plants begin producing then after the initial wait for your first plants to be harvested you ought to be able to harvest a yearly crop of these unfortunately uncommon yet exceptionally tasty and versatile roots.

*I am choosing to write about this now because a) I believe it to be relevant and b) because this is an botanical fallacy that is commonplace yet is still inexcusable. Sweet potatoes and yams are not the same thing. They are not the same species either and aren’t even closely related as they belong to two distinct botanical families, the Convolvulaceae and Dioscoreacea, respectively. Although they may look the same at times, they come in a wide range of different sizes, skin colors, flesh colors, have different flavors and textures, can be used in different ways and originate from different regions of the world. The reasons for this are a bit much for me to try and cover, but if you’re interested you can read more about it here.

**It is worth noting that these bulblets, if not deliberately collected with the intent of either consuming them or deliberately introducing them to new sites, are quite capable of distributing themselves. Although it is not known whether or not this species has the capacity to escape cultivation in the milder regions of Canada where it can be grown, it is still probably best to err on the side of caution and pay attention to try not to let this yam get away from you.